1. Field of the Invention
The present invention relates to a distributed feedback semiconductor laser used in optical communication.
2. Description of the Background Art
In a conventional phase-shift distributed feedback semiconductor laser for optical fiber communication, when the cycle of a diffraction grating is represented by Λ, single-axial mode oscillation has been realized by a Λ/2-phase-shift structure or the like. However, in this structure, the intensities of laser beams output from the front and rear end faces are almost equal to each other. For this reason, in order to obtain a large optical output from the front end face, a large drive current must be applied to the semiconductor laser.
In order to solve the problem, an asymmetrical structure may be given to the diffraction grating to achieve an efficient activity distributed reflective laser (for example, see reference 1).
This attempts to achieve high efficiency such that a Λ/2-phase-shift structure or the like is arranged between a region on the rear end face side and a region on the front end face side to obtain a single-axial mode. It is assumed that a coupling coefficient of a diffraction grating in the rear end face region is represented by κ1 and that a coupling coefficient of a diffraction grating in the front end face region is represented by κ2. In this case, the diffraction grating in the front end face region has a corrugation which is shallower than that of the diffraction grating in the rear end face region. For this reason, an optical output P2 from the front end face of the front end face region is larger than an optical output P1 from the rear end face in the rear end face region. This is because a ratio (A2/A1) of a power A2 of a lightwave emitted from a phase-shift region to the front end face to a power A1 of a lightwave emitted to the rear end face increases depending on the depth of the asymmetrical corrugation. When a concrete parameter such as a dimension is given to the laser, a large optical output ratio (P2/P1) of 1 to 16 or 1 to 27 can be obtained.
However, in the laser described above, as the ratio of κ1/κ2 is increased to increase the optical output ratio (P2/P1) of the front end face and the rear end face, a threshold gain difference Δgth between a main axial mode and a sub-axial mode becomes small. In high-speed modulation, oscillation is easily made in the sub-axial mode disadvantageously.
In the distributed feedback semiconductor laser, in order to improve the linearity of an optical output/current characteristic, the coupling coefficient is changed in the direction of resonator length (for example, see reference 2).
This Reference 2 aims at improving the linearity of an optical output/current characteristic. Therefore the coupling coefficient is changed in the direction of resonator length. But the concrete value over coupling coefficient is not described.
[Reference 1]
Eda et al., IEICE electric wave section meeting lecture letters in October 1984, No. 271 in the second separate volume
[Reference 2]
Japanese Patent Application Laid-Open No. 10-223967 (1998)